I think Swift UI's more concrete problem is that it just doesn't have a sufficiently mature enough API with the right mix of low level primitives and higher abstractions.
Say whatever you want about modern web development, but I think React/JSX + HTML/CSS has proven you can use a declarative model to build complex user interfaces.
This could be taken to support my point. There is a MASSIVE amount of literature on the internet about how to do this, and it's reasonably straightforward to see what is happening with the DOM when you use these frameworks. Likewise, a novice developer can create something that causes scrolling to stutter or other performance problems pretty easily in this world.
Take on the other hand, some of SwiftUI's more advanced features, like PreferenceKey which are extremely under-documented (the generic sounding name doesn't help either). Of course folks are going to struggle to do more complex things.
I wonder how much the situation would be helped if the thing you searched for help allowed in-line SwiftUI examples, like you can do on the web. Playgrounds is kind of that thing but you aren't searching a repository of playgrounds for documentation.
I think the web developer's toolkit has benefitted from competition - every browser prioritizes supporting dev tools because that's the only way to get more sites to support your particular quirks. people building web frameworks competed for developers ruthlessly, because starting to learn a new framework is only a few search queries away, and a great deal of effort has been made to improve the overall developer experience.
Say whatever you want about modern web development, but I think React/JSX + HTML/CSS has proven you can use a declarative model to build complex user interfaces.